An integrated circuit (IC) or semiconductor die may be packaged as a leadless package whereby the IC or die is first attached to a leadframe, followed by electrical interconnection established between the die and the leads of the leadframe, and the interconnected die is then encapsulated with plastic to a specific package outline before the packaged IC is singulated from the leadframe. The singulation process involves cutting to sever the leads, leaving external leads to protrude or pads exposed from the package as the electrical contact points for the IC. The singulation process may involve mechanical punch-and-die cutting means, sawing means, chemical etching or combinations thereof.
For a leadless package, the leads are cut as close as possible to the package body so that an outline or footprint as small as possible is obtained in order that the IC package occupies as little area as possible on a printed wiring board which population density is ever increasing due to demands of miniaturisation and compact product design. In this specification, the term “lead” may include tie bars and other like projections that extend out of the mould (during encapsulation) or the package (after encapsulation).
In the conventional encapsulation process, as shown in FIG. 1 (Prior Art) in plan view, the lead webbing of the leadframe (10) enables the interconnected die or IC to be held in accurate alignment within the moulding cavity (12) of the encapsulation mould which typically comprises a top mould-half and a complementary bottom mould-half. The closure of the mould-halves would still leave a gap at the parting line due to the leads extending from the moulding cavity. The gap enables the molten resin, which is injected into the cavity (12) via gate (13) to fill up the cavity (12), to overflow and seep out through the gap (thus forming flashes and bleeds) until it is stop by a dam bar (14) or rail (15) which extends traversing the leads (16) such as disclosed in U.S. Pat. No. 5,623,162 (NEC Corporation).
Upon curing of the resin and parting of the mould-halves, a broad flange (18) is formed around the package body (20) as shown in shadings in FIG. 2 (Prior Art). The broad flange has a thickness similar to the gap due to the leadframe's thickness. The next step is nicking to provide cut lines (22) as close as possible to the package body (20) to form the perimeter of the package outline so that the resultant singulated package has a base or bottom outline as small as possible. Cutting through the broad flange (18) in the singulation process thus involves cutting through both the resin layer and the leads in a single shearing action. The singulated package is shown in FIG. 3 (which is also the object of the moulding of our present invention). Due to the aforesaid demands of small package outline, the shearing close to the package body often results in a higher percentage of package rejects due to chipping, cracking or delamination defects as a result of the high stress of cutting through the leads and the moulded resin between the leads. Also mentioned in this NEC patent (at col. 6 lines 33-36) is the factor played by the moulding deviation order in the resin encapsulating and curing steps (e.g. in the order of 0.05 mm) that will result in the lead protrusion in a certain range of tolerance (i.e. 0-0.25 mm for the above example).
As such it would be desirable to design a mould which could limit the overflow or seepage of the resin through the gap in between the mould-halves' parting line due to the leads' (16) extension out of the moulding cavity (12). Preferably, the resin flow is limited to the cutting line so that only the leads (16) and tie bars (17) need to be cut during singulation. In this manner, the shearing close to the package body will only need to cut through the leads, i.e. without the moulded resin between the leads. There may be a number of patents on the method or devices for stopping, limiting or controlling the flow of molten resin out of the mould during encapsulation—however, we have not been able to find any that limits the resin flow by providing modifications to the mould to achieve this object. The following is representative of the various patents on methods and devices which are different from our invention.
U.S. Pat. No. 5,328,870 (Amkor Electronics) discloses a mould design with a dimensional mismatch between a heat sink's thickness and the corresponding mould cavity, so that the encapsulant is prevented from forming on the exposed heat sink surface. This technique is not applicable in our case as it is designed for encapsulating and affixing a heat sink to the top of the package.
U.S. Pat. No. 5,897,883 (Samsung Electronics) discloses modifications to the mould cavity that are thought to reduce the difference between the shear velocities at the edges and at the centre of the cavity. The modifications comprise providing continuous projections (261) along the lower edges of the bottom mould-half in the flow of the resin. Various profiles of the projection (261, 261a, 261b) are disclosed, including one which may be inserted as a separate projection block (261c). These projections are provided within the surfaces of the mould cavity to control the encapsulant sweep or flow and are not designed for stopping the resin flow at the periphery of the package as in our invention.
U.S. Pat. No. 6,627,976 (Amkor Technology) discloses a mould design whereby a gate and multiple exit vents for the resin flow are provided. They are formed by sills (48) which project downwardly from the upper mould-half. The sills (48) are formed on the mould upper half's lower surface and is a continuous structure along the dam bars and rail of the leadframe except for the corners of the package where exit vents are provided. The sills serve to narrow down the resin flow path into the mould cavity and therefore correspond to the function of a gate in addition to the mould gate (152), and as well as at the vents (154, 156 and 158). The resin will only stop at the dam bar as in conventional methods and the resultant package thus has a broad flange which requires cutting to size.